Update.
The sensor installed, tested (no signal when it sees water, but when i have lifted close to the top of the tank, above the water level, it gives 12V), seems to be OK!
I have wanted to test it by removing water from the expansion tank but could not do it on an easy way (I have wanted to put a hose from top but could not) so I will do it next time when I have to work on cooling system (or on something for which I have to open it).
The sensor installed, tested (no signal when it sees water, but when i have lifted close to the top of the tank, above the water level, it gives 12V), seems to be OK!
I have wanted to test it by removing water from the expansion tank but could not do it on an easy way (I have wanted to put a hose from top but could not) so I will do it next time when I have to work on cooling system (or on something for which I have to open it).